A variety of fastener means utilizing the attraction of a permanent magnet has been known, and each differs in the structure depending on the use.
As one typical example of a fastener means for handbags, etc., there is known a magnetic lock closure for baggages and satchels disclosed in the Japanese Utility Model Publication No. Sho 56-45985.
This prior art lock closure uses a disk-like permanent magnet having a through-hole in the direction of the magnetic poles. The permanent magnet is housed in a plate-like casing. An attracting member is formed by placing a ferromagnetic plate having a ferromagnetic projection within said casing, with the ferromagnetic projection extending in said through-hole and the ferro-magnetic plate being in contact with the plane of a magnetic pole of said permanent magnet. A member to be attracted within the through-hole of the permanent magnet constituting the attraction member comprises a ferro-magnetic projection which abuts against and is attracted by the projection of the attraction member and a ferromagnetic plate which is attracted to the surface of the attraction member.
One of the magnetic poles of the permanent magnet of the attraction member is attached with a ferromagnetic plate, while the other magnetic pole attracts a ferro-magnetic plate that constitutes the attracted member in the prior art lock closure. The magnetic force converged on the ferromagnetic plates of the attraction and attracted members forms a closed circuit as it passes the respective ferromagnetic projections located inside the through-hole. The lock closure of this construction features a higher efficiency of attraction as compared with the fastening means of other constructions utilizing a permanent magnet. However, the permanent magnet of the attraction member is formed like a disk in the prior art lock closure, and its surfaces at the magnetic poles and its peripheral side face between the poles form substantially a right angle. As a result, the magnetic poles of the permanent magnet are arranged at the shortest interval distance for its thickness.
It is generally known that the magnetic flux of a permanent magnet connects the two magnetic poles with a circuit with the least reluctance. When ferromagnetic projections are interposed between the two surfaces of the magnetic poles, as is the case of said lock closure, much of the magnetic flux becomes converged on the projections.
However, it is also generally known that when a permanent magnet is arranged at a position away from said projections as in the prior art lock closure, the magnetic flux along the peripheral edge of the respective magnetic pole surface forms a magnetic path along the peripheral side of the magnet between the magnetic poles as a path with a magnetic reluctance lower than that of the path leading to the projections.
The prior art lock closure is defective in that the magnetic flux on the peripheral side of the permanent magnet does not contribute to the attraction force of the lock closure; rather, it tends to destroy the information magnetically recorded on magnetic tickets, etc.
Because the permanent magnet used in the lock closure has the minimum distance between the magnetic poles for its thickness, considerable leakage flux occurs on the peripheral side, weakening the attraction force of the lock closure by the amount of this leakage flux.